HORSESHOEING, 587 
as a soft and very elastic wedge between the bars and between the 
edges of the sole just in front of the bars. A broad and shallow de- 
pression in its center divides it into two branches, which diverge as 
they pass backward into the horny bulbs of the heel. In front of the 
middle cleft the two branches unite to form the body of the frog, 
which ends in the point of the frog. The bar of a bar shoe should 
rest on the branches of the frog. In unshod hoofs the bearing edge 
of the wall, the sole, frog, and bars are all on a level; that is, the 
under surface of the hoof is perfectly flat, and each of these structures 
assists in bearing the body weight. 
With respect to solidity, the different parts of the hoof vary 
widely. The middle layer of the wall is harder and more tenacious 
than the sole, for the latter crumbles away or passes off in larger or 
smaller flakes on its under surface, while no such spontaneous short- 
ening of the wall occurs. The white line and the frog are soft-horn 
structures, and differ from hard horn in that their horn cells do not 
under natural conditions become hard and hornlike. They are very 
elastic, absorb moisture rapidly, and as readily dry out and become 
hard, brittle, and easily fissured. Horn of good quality is fine 
grained and tough, while bad horn is coarse grained and either 
mellow and friable or hard and brittle. All horn is a poor con- 
ductor of heat, and the harder (drier) the horn the more slowly 
does it transmit extremes of temperature. 
THE PHYSIOLOGICAL MOVEMENTS OF THE HOOF. 
A hoof while supporting the body weight has a different form, and 
the structures inclosed within the hoof have a different position than 
when not bearing weight. Since the amount of weight borne by a 
foot is continually changing, and the relations of internal pressure 
are continuously varying, a foot is, from a physiological viewpoint, 
never at rest. The most marked changes of form of the hoof occur 
when the foot bears the greatest weight, namely, at the time of the 
greatest descent of the fetlock. Briefly, these changes of form are: 
(1) An expansion or widening of the whole back half of the foot 
from the coronet to the lower edge of the quarters. This expansion 
varies between one-fiftieth and one-twelfth of an inch. (2) A nar- 
rowing of the front half of the foot, measured at the coronet. (3) \ 
sinking of the heels and a flattening of the wings of the sole. These 
changes are more marked in the half of the foot that bears the 
greater weight. 
The changes of form occur in the following order. When the foot 
is set to the ground the body weight is transmitted through the bones 
and sensitive and horny leaves to the wall. The coffin bone and 
navicular bone sink a little and rotate backward. At the same time 
